Apparatus for reducing bias in telegraph signals



Dec. 28, 1943. K. w. RICHARDS APPARATUS FOR REDUCING BIAS IN TELEGRAPH SIGNALS Filed Dec. 3, 194].

UUxSSW SSV N T/f/r/uwww BY A a2 a fi ATTORNEY f Patented Dec. 28, 1943 OFFICE APPARATUS FOR.EEDUCING BIAS IN TELE-.

GRAPH SIGNALS 7 Application December 3, 1941, Serial No. 421,415

6 Claims.

This invention relates generally to telegraph systems and, more particularly, to teletypewriter systems. This invention also relates to arrangements for reducing bias in the signals received in telegraph or teletypewriter systems.

It has been found that in telegraph subscriber setswhich may include simple telegraph repeaters together with other equipment for receiving signals from and sending signals to the switchboard of a central ofiice there occur substantial voltage variations in the rectified power supplied to the apparatus. These voltage variations introduce biasin the signals received by the receiving relay from the line. desirable to have the telegraph subscriber sets and like equipment made substantially independent of such power voltage variations, and thereby substantially reduce the bias and the consequent distortion in the signals received by the system.

It is, therefore, one of the objects of this invention to reduce bias in the signals. received over a telegraph circuit. This may be accomplished, according to the present invention, by so connecting a non-linear structure to the system that the effect of substantial voltage variations will be reduced and practically nullified.

In one specific embodiment of the invention to be described hereinafter for illustrative purposes, a structure having a non-linear characteristic will be. connected to the bias winding of the receiving relay of the telegraph system, and it.

Will be so arranged and proportioned that as the rectified voltage varies, the current traversing the bias winding of the relay will be adjusted automatically to substantially reduce the signal bias which ordinarily accompanies such voltage variations. This will then result in more accurate signals being received by the other equipment of the system.

This invention will be better understood from the more detailed description hereinafter following, when read in connection with the accompanying drawing showing one embodiment of the invention merely for the sak of. illustration.

Referring to the drawing, there is shown a line L interconnecting a simple switchboard repeater circuit with a telegraph subscribers set which is arranged for the transmission and reception of teletypewriter signals. The switchboard repeater circuit may include, among other things, a sending relay W1 and. a receiving relay W2, the line winding of which is connected between the armature of the sending relay W1 and the line L. The line L is. shown connected It would, therefore, be

at the subscribers set to the rectifier D over a circuit which includes a variable resistor Z1, the upper winding of a receiving relay W3, another resistor Z2, a coil K and the armature and marking contact M of a pole-changing relay PCi which extends to the negative terminal of the rectifier l). The positive terminal of the rectifier D is connected to ground through the armature and marking contact M of another Pole changing relay PS2. The rectifier D may be'of any wellknown type, and is employed to convert alterhating current supplied from a suitable source into a direct current potential which is intended to be approximately of the proper magnitude. The lower or bias winding of the receiving relay W3 is connected in a circuit which includes a variable resistor Z3, the rectifier D, the armature and marking contact M of the pole-changer P02 and ground. A non-linear device Eta-which may be a Rectox unit or a thermistor-i connected across the lower or bias winding of relay W3.

The device R2 is preferably one of large impedance employed. to' prevent sluggishoperation' of relay Ws. I

The teletypewriter apparatus at thesubscribers set includes, among other things, sending equipment designated TTY1 and receiving equipment designated TTYz. The sending equipment TTYi' is connected to the negative terminal of rectifier D through the upper windings of the pole-chang- The other side of the teletypewriter equipment TTYiing relays PC2 and PCI and the resistor Z4.

is connected to the positive terminal of rectifier D. The lower or bias windings ofrelays PCI and PC2 are connected to rectifier Din aserie s circuit which includes impedances Z4 and Z6 in addition to these windings. ing from the sending equipment 'ITYi traverse the upper windings oi relays PC]. and PCz, the armatures ofthese relays will reverse the potential supplied by rectifier D to line L in accord"- ance with the signals, as will be pointed out hereinafter.

The receiving portion TTYz oi the teletypewriter has its lower terminalconnected-to ground through a rectifier R3 and the armature and As signals emanat'-' The lower or bias winding of receiving relay W3 carries a current of such magnitude as to.

maintain the armature of the relay .Ws on its marking contact when a teletypewriter marking signal is transmitted from the subscribers set over the line L to the'switchboa'rd repeater circuit. When a spacing signal is' transmitted from the subscribers set, the bias winding of relay W3- will carry practically no current, but the line current traversing the upper winding of relay W3 will be in a direction to hold its armature on its contact M. ,However, when signals are re ceived from the switchboard repeater circuitover line L, the current in the upper or main winding of relay W3, will act to overpower the efiect of the current in the lower'or bias winding so that the relay W3 will respond to the received signals and its armature will then repeat the received signals into the teletypewriter receiving equip ment TTYz.

The condenser C1 and the resistor Z are connected between the lower or bias winding of the relay W3 and the left-hand terminal of the coil K so as to balance'the line L in the manner of an artificial line against the effects of transient or other extraneous voltages. The lower windings of the'pole-changing relays PCI and PC2 are shown connected in series with each other and with a resistor Zs, these windings'being the usual bias windings for such relays.

When signals are transmitted from the tele-' typewriter sending equipment TTYi over the line L to the distant switchboard repeater circuit, the pole-changing relays PC]. and P02 Will respond to these signals, the armatures of these relays closing their marking contacts M with each mark ing signal and their spacing contacts S with each spacing signal. Each marking signal will cause the negative terminal of the rectifier D to be connectedto the line L over a circuit which includes the marking contact M and armature of relay PCi, coil K, resistor Z2, the upper winding of relay W3 and the resistor Z1. 'At the same time the positive terminal of rectifier D will be grounded through the armature and marking contact M of relay PCz. When the armatures of the relays P01 and PC2 close their spacing contacts with each spacing signal coming from the teletypewriter equipment TTY1, the voltage applied to line L at the subscribers set will be reversed so that the positive terminal of the rectifier D will be connected to the line L over a circuit which includes the spacing contact S and armature of relay PCi, coil K, resistor Z3, the

upper winding of relay W3 and resistor Z1. At'

.Signals transmitted by the sending equipment;

TTY1 of the subscribers set will'al'so' be recorded by the teletypewriter receiving equipment 'I'TY'z' to provide the home copy. Each marking signal will cause the negative terminal of the rectifier D to be connected through the marking contact M and armature of relay PCI to the upper terminal of the winding of the magnet of the receiving equipment 'ITYz, the lower terminal of which is connected to ground through the rectifier Rsand the armature and marking contact M of relay W3. The positive terminal of the rectifier D will at the same time be connected to ground, as already described. Hence each marking signal will result in a flow of current through: the magnet of the receiving teletypewriter equip-- Each spacing signal will reverse: "the polarity of the rectifier D with respect to the magnet winding of the receiving teletypewriter- .ment TTYz.

' supplied by rectifier D to the circuit of the receiving magnet TTYz and rectifier R3, and the receiving apparatus will register the signals.

Signals way be received at the subscribers set from line L in the usual way. The sending relay *W1 will move its armature between its M and S contacts so as to apply to the line L either ground or the positive potential of, for example, a battery as shown in the drawing. The potential of ,the battery may exceed that derived from the rectifier D. All such signals from the switchboard circuit'will, as already explained, cause the armature of the relay W3 to move between its M and S contacts, and hence the signals will be repeated to the receiving teletypewriter equip- 40" ment TTY2.

' shown bridging the bias winding relay W3.

linear network will change in impedance and, therefore, change the current through the bias winding of relay W3. As the voltage supplied by the rectifier D increases, the impedance of the device R3 will be reduced, and conversely, as the voltage supplied by rectifier D is reduced, the impedance of the non-linear device R2 will be increased. The non-linear device R3 is of such impedance that with changes in the terminal voltage of rectifier D within certain limits, the bias current will change non-linearly to reduce bias forming to this invention, the rectifier R2 circuit served to maintain the bias current in the lower winding of relay W3 at approximately half the sum of the marking and spacing currents in'the upper winding of relay W3 and hence signal bias was practically eliminated.

The non-linear device R2 may comprise, for example, a large number of copper oxide disks of small diameter, or it may comprise one or more elements of thyrite or one or more thermistors of well known type. It is, a structure which while carrying current undergoes impedance changes with changes in the voltage of rectifier D, thereby changing the current in the bias in the signals received. In one arrangement con-' winding of relay W3 according to a predetermined relationship. This predetermined relationship involves a current in the bias winding of relay W3 which is closely equal to one-half the sum of the marking and spacing currents in the upper or main winding of relay W3, as already noted hereinabove.

In place of impedance element such as R2 in shunt with the bias winding of relay W3, the impedance of element R2 varying negatively with changes in the rectified voltage, a non-linear impedance element may be connected in series with the bias winding of relay W3, the series impedance element varying positively with changes in the rectified voltage. Such a non-linear series impedance may be employed in addition to impedance Z3 or the impedance Z3 may itself comprise the positive non-linear series element, it so desired. The positive non-linear series element will therefore vary in the same direction as the current in the bias winding changes. The positive non-linear impedance element will preferably be so proportioned as to maintain the current in the bias winding at substantially onehalf the algebraic sum of the marking and spacing line currents flowing through the upper winding of relay W3.

The rectifier R3 is shown shunted by a condenser C2. This is optional in the system, but it may be added for the purpose of by-passing transients and other extraneous voltages around the rectifier.

Although the invention has been described as applied to an'arrangement employing a rectified source of .power voltage, it will be understood that the invention may also be applied to circuits having batteries or generators or the like for supplying the necessary D. C. voltage to the local circuits.

While this invention has been shown and described in certain particular embodiments merely for the purposer of illustration, it will be understood that the general principles of this invention may be applied to other and widely varied organizations without departing from the spirit of the invention and the scope of the appended claims.

What is claimed is:

1. In a signaling system, the combination of apparatus for maintaining a predetermined ratio between the currents supplied to first and second circuits, the first circuit being supplied with current from a first source of potential while both circuits are simultaneously supplied with current from a second source of rectified variable potential, said apparatus including an impedance element connected in series between said second circuit and said second source, and an impedance device having a non-linear impedance characteristic connected across said second circuit.

2. In a signaling system, the combination of a source of rectified potential the magnitude of which varies, a first circuit connected to said source, a second circuit supplied with current from said source, and means for maintaining a predetermined ratio between the currents traversing said first and second circuits while the magnitude of said potential varies, said means including an impedance device connected serially between said second circuit and said source, and a non-linear impedance device also connected to said second circuit.

3. In a signaling system, the combination of apparatus for maintaining the currents traversing first and second circuits in a predetermined ratio, said apparatus including a source of rectified potential connected to said first circuit, the potential of said source being variable, an impedance interconnecting said second circuit and said source, and a non-linear device having a negative impedance coeificient connected across said second circuit.-

4. In a telegraph system, the combination of a line over which signals may be transmitted, a relay having first and second windings, a source of rectified potential, the first winding of said relay being connected between said line and said source of rectified potential so that signals received over said line may be registered by said relay, an impedance element connected in a series circuit including said second relay winding and said source of rectified potential, and a non-linear device having a negative impedance charactertistic connected across the circuit of said source of rectified potential and said impedance element.

5. Apparatus for regulating the current in a translating device having first and second circuits so that the current in the second circuit is a predetermined ratio of the sum of the currents in both circuits, the second circuit being connected to a source of variable rectified potential, comprising an impedance element connected in series with said source of variable potential and said second circuit, a non-linear device connected in shunt across the circuit of said source of potential and said impedance element, said non-linear device being reduced non-linearly in impedance as the potential of said source increases and increased non-linearly in impedance when the potential of said source is reduced.

6. A telegraph station comprising a line over which telegraph signals may be transmitted, a receiving relay at said station including first and second windings, sending apparatus at said station, a source of variable rectified potential connected through said first winding to said line, said source of potential being also connected to said sending apparatus, means responsive to signals from said sending apparatus to alternately reverse the potential of said source applied through said first winding of said receiving relay to said line, receiving apparatus controlled by the armature of said receiving relay, a non-linear device, an impedance connected in series with said second winding of said receiving relay and said source or" potential, the non-linear device being connected in shunt across the circuit of said source of potential and said impedance, the

impedance of said non-linear device varying inversely with variations in the potential of said source.

KENNETH WATSON RICHARDS. 

